This invention relates generally to transceiver architecture in a wireless portable communication device.
Radio frequency (RF) transmitters are found in many one-way and two-way communication devices, such as portable communication devices, (cellular telephones), personal digital assistants (PDAs) and other communication devices. An RF transmitter must transmit using whatever communication methodology is dictated by the particular communication system within which it is operating. For example, communication methodologies typically include amplitude modulation, frequency modulation, phase modulation, or a combination of these. In a typical global system for mobile communications (GSM) mobile communication system using narrowband time-division multiple access (TDMA), a Gaussian minimum shift keying (GMSK) modulation scheme is used to communicate data.
The deployment of new wireless systems presents unique challenges to mobile handset designers. In order to reap the full benefit of expanded capacity and increased data bandwidth, the new handsets must work on both the new systems as well as the old. One of these new systems has been named Enhanced Data Rates for GSM Evolution (EDGE). The EDGE standard is an extension of the Global System for Mobile Communications (GSM) standard.
The EDGE standard increases the data rate over that available with GSM by sending more bits per RF burst. More bits are sent in EDGE by using a modulation scheme based on 8-phase shift keying (8-PSK), which provides an increase over GSM's Gaussian minimum shift keying (GMSK) modulation format. In the EDGE modulation scheme, the 8-PSK constellation is rotated 3 radians every symbol period to avoid problems associated with zero crossings. In contrast to GMSK's constant amplitude envelope, the added rotation factor in the EDGE modulation scheme results in a non-constant amplitude envelope. The polar modulation scheme can be described in terms of amplitude modulation (AM) and phase modulation components. This non-constant AM presents some difficulties with regard to RF power control. These problems are exacerbated by the desire to have a single transmitter that can be used for both the GSM and EDGE standards. Conventional architectures use a bias controller that tracks the AM envelope to control a RF power amplifier. The PM component further controls the RF amplifier. The bias controller is essentially a low drop-out (LDO) regulator that drops the battery supply voltage by behaving like a resistor. Accordingly, conventional bias controllers increase the power budget of the mobile handset, which results in reduced talk times.
Previous solutions for dynamic power control in a mobile handset have focused on handsets that use the code division multiple access (CDMA) communication standard. CDMA systems use a complex modulation scheme that involves multiple users communicating on the same 1.25 MHz channel at the same time. An individual user is distinguished by a digital code while users with different codes appear as white noise. This noise level gradually increases as the number of users on a specific channel increases.
In CDMA systems, the variation in the number of users on a single channel in conjunction with the motion of a handset throughout a large cell area causes very large variations in the power levels available at the handset antenna. When the handset is at the edge of a cell site using a channel with many users, a very low noise figure is necessary to detect the required signal from the surrounding noise. On the other hand, when the handset is very close to the base station, a high input third-order intercept (IP3) point is essential to prevent distortion of the signal by non-linearities in the receiver. To meet these requirements, CDMA handsets need a receiver with a very wide dynamic range.
Previous solutions for dynamic power control in a CDMA handset track the envelope of the RF signal. That is, a DC-DC converter tracks the envelope of the RF signal. Although this approach maximizes power amplifier efficiency, it has some disadvantages. First, the DC-DC converter must be a high-bandwidth device to closely track changes in the RF envelope. A high-bandwidth DC-DC converter requires a relatively high-switching frequency to limit output ripple, which reduces the power efficiency of the DC-DC converter. In addition, a solution that tracks the RF signal requires precise alignment of the phases between the AM path and the PM path of the transmitter or a mechanism to counter any phase shift introduced by the DC-DC converter in the AM path.
Therefore, it would be desirable to provide dynamic power control in a mobile handset in an economic and efficient manner absent constraints on bandwidth, switching frequency, and alignment of amplitude and phase modulation paths in a transmitter.